Abstract

The circulation of the North Atlantic Deep Water (NADW) and tracer fluxes in the South Atlantic are determined from hydrographic, nutrient, and tracer (terrigenic helium, freon-11, freon-12 and carbon tetrachloride) data from WOCE and other high quality pre-WOCE sections using a linear box-inverse model. Multiple linear regression, which makes use of the correlation between different parameters, is applied to infer the missing parameters in the bottle data set. This interpolation technique also enables us to include the nutrient and tracer measurements in the inverse model with a spatial resolution of the corresponding CTD data. Only quality-checked profiles are used that reach the bottom and are positioned continuously along a section. These data define a set of 126 closed boxes on which conservation requirements are imposed. A detailed water mass analysis is performed, incorporating the tracer information from the whole South Atlantic, to determine the vertical boundaries of these boxes. As a result the water column is divided into 11 layers which are defined by neutral densities.

Constraints for the inverse model are an integral phosphorus transport, the overall salt and silica conservation, as well as flow conditions inferred from moored current observations. Additionally it is assumed that the meridional salt transport in the Atlantic is constant and equals the transport through Bering Strait. This acounts for the effect of evaporation, precipitation and runoff.

The results of the inverse model are analysed with an emphasis on the zonal spreading of the NADW. A clear meridional separation in the direction of the zonal NADW transports can be observed. The cumulative transports show an eastward flux of relatively fresh NADW in the Brazil Basin between 20°S and 25°S. This flow begins near the Vitoria-Trinidade Ridge and continues to the Rio de Janeiro Fracture Zone of the Midatlantic Ridge. To the north of this eastward motion, between 10°S and 20°S , older NADW returns back to the west. In the Angola Basin an intensification of the general eastward flow appears in the upper NADW layers near 25°S. This flow is directed toward the Walvis Ridge and is stronger and located more to the south than the Namib-Col Current described by Warren and Speer (1991) at 22°S .

The meridional freon fluxes are directed to the north through all zonal sections and decrease generally from the south to the north. The values of the freon-12 fluxes are between 20 mmol/s in the south and 10 mmol/s in the north. Those of the freon-11 are in a range of 20-40 mmol/s. Through the A11 section at 45°S the resulting freon-11 flux is smaller than the value of Saunders and King (1995b) by a factor of 1000. Most probably this is just an error in the reported units. An estimate of the magnitude of the freon-11 flux calculated from the strength of the overturning cell and the freon-11 concentrations in the upper layers confirms the smaller values from this study.

The freon-11 fluxes in the CFC-rich AAIW layer support the hypothesis of an anticyclonic basin-wide recirculation of the intermediate water. Also the CFC-flux in the AABW und NADW layers agrees well with the known spreading patterns. Generally the inspection of the tracer fluxes in the different %layers advances the interpretation of the water mass spreading. layers improves the understanding of the water mass spreading. Due to the difference in the tracer concentrations of different water masses in the same density class, the circulation of a water mass is better described by tracer fluxes than by the volume fluxes.

The infered meridional flux of terrigenic helium is directed southward. This implies a source of terrigenic helium in the Atlantic as also seen be Rueth (1998).